The present invention relates to a mounting substrate with a solder resist layer and a method of forming the same, and more particularly to a mounting substrate with a solder resist layer and with electrode pads on the substrate such as a printed wiring board and bumps including solder pre-coat on the electrode pads.
The screen printing method has been the majority method of forming the mounting substrate with a solder resist layer and with electrode pads on the substrate such as a printed wiring board and bumps including solder pre-coat on the electrode pads.
There are the following four methods of forming solder bumps. The first method is a plating method, where electrode pads are exposed to an electroless solder plating solution to apply a solder on surfaces of the electrode pads for subsequent fusing the same. The second method is a solder dip method, where a solder is supplied onto the surfaces of the electrode pads by a hot air leveler method. The third method is a paste printing method or a super solder method, where a printing mask such as a tetron screen, a metal mask, or a plastic mask is used to supply a solder paste by printing method for subsequent curing the same. The fourth method is a solder powder method, where solder powders are sprinkled onto an adhesion layer formed on pad portions to which a solder is to be supplied. This fourth method is disclosed in Japanese laid-open patent publication No. 7-74459.
The above four conventional methods have the following disadvantages. A large variation in amount of supplied solder onto each electrode pad is unavoidable. As a pitch of electrode pads is made narrower, a bridge may be formed between adjacent two pads.
In Japanese laid-open patent publication No. 4-173385, it is disclosed that a squeegee is used to print a cream solder through a film mask adhered on a substrate surface for subsequent removal of the used film mask.
In Japanese laid-open patent publication No. 6-350230, it is disclosed that on a first solder resist layer, a second solder layer is formed which is narrower than the first solder layer and then a solder resist layer is formed between electrode pads, where the solder resist layer is formed thicker than the electrode pads to prevent formation of any solder bridge.
In Japanese laid-open patent publication No. 10-350230, it is disclosed that a gap is formed between a solder dam and an electrode pad to reduce a percent defective in mounting the device
The above Japanese publications are, however, silent on how to prevent formation of the solder bridge between adjacent pads having a fine pitch wherein bumps are formed on the electrode pads which are formed on a mounting substrate with a solder resist layer.
In the above circumstances, it had been required to develop a novel mounting substrate with a solder resist layer free from the above problem.
Accordingly, it is an object of the present invention to provide a novel mounting substrate with a solder resist layer electrode pads on the mounting substrate and solder or metal bumps on the electrode pads free from the above problems.
It is a further object of the present invention to provide a novel mounting substrate with a solder resist layer electrode pads on the mounting substrate and solder or metal bumps on the electrode pads at a small pitch without any bridge between adjacent bumps.
It is a still further object of the present invention to provide a novel method of forming a mounting substrate with a solder resist layer electrode pads on the mounting substrate and solder or metal bumps on the electrode pads free from the above problems.
It is yet a further object of the present invention to provide a novel method of forming a mounting substrate with a solder resist layer electrode pads on the mounting substrate and solder or metal bumps on the electrode pads at a small pitch without any bridge between adjacent bumps.
The present invention provides a substrate structure comprising: a plurality of electrodes provided on a substrate, each of the electrodes comprising an electrode pad on the substrate and a bump on the electrode pad; and a solder resist layer provided over the substrate, so that the solder resist layer extends in gaps between the electrodes and covers the electrode pads and the bumps except for at least top portions of the bumps, whereby top portions of the bumps are positioned higher than a surface of the solder resist layer to have the solder resist layer isolate adjacent two of the electrodes.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.